Many therapeutic molecules form covalent bonds with cysteine residues on their protein targets. The mechanisms of the majority of these molecules were either elucidated long after development or are not fully understood. Recent successful drug discovery efforts, however, moved to structure-based design. These require both an accurate structural model of the target protein and a high-specificity ligand.
One third of therapeutic molecules, including many blockbuster drugs, form covalent bonds with their targets. These electrophilic drugs generally bond to a nucleophilic amino acid, often serine or cysteine, on a target protein. Aspirin and penicillin (and their many derivatives) acylate serines and numerous drugs form covalent bonds with specific cysteines. These therapeutic agents are effective despite the potential for off-target reactions with hundreds of highly reactive, nucleophilic residues, which are often required for the function of essential proteins. A worst case scenario for reaction with the “wrong” nucleophile is nerve gases (e.g., Sarin, intravenous LD50 ˜30 μg/kg), which covalently modify the active site serine of acetylcholine esterase. On the other hand, comparable toxicity has been harnessed to selectively target cancer cells—bortezomib/Velcade (LD100<250 μg/kg) selectively modifies an active site threonine of the proteasome. Unintended reaction with a highly reactive nucleophile isn't necessarily disastrous—it has led to a drug. The disulfide-containing substance, disulfiram, was intended to treat parasitic infections, but when tested on humans gave severe “hangover” symptoms upon alcohol consumption. Years after its therapeutic use began, this compound, dubbed antabuse, was found to bind the highly reactive active site cysteines of alcohol dehydrogenase. Nevertheless, the paucity of therapeutic suicide inhibitors to most human proteases, which (unlike viral proteases) have numerous homologues with identical off-target catalytic sites, has been attributed to off-target nucleophiles.
With effective covalent drugs, off-target binding tends to be offset by selectivity for the target and the enhanced potency inherent to irreversible inhibition. The uncanny specificities of cysteine-binding therapeutics involve elegant and usually serendipitous chemistry. The gastroesophageal reflux disease drugs (GERD, e.g., omeprazole/Prilosec™, lansoprazole/Prevacid™, etc.) use a cyclic sulphenamide to irreversibly bind a cysteine residue of the proton pump of the intestinal lumen. These benzolamide-derivative prodrugs require protonation of a low pKa pyridine nitrogen (pKa<4.5) for activation and sequestration. They are neutral, inactive, and permeable, but are activated upon encountering the pH˜0.8 parietal cell canaliculus, which contains their target (i.e., the proton pump). Here, they accumulate at 1000-fold higher concentrations. While the chemical basis of proton-mediated accumulation of omeprazole was appreciated, if not designed, the elegant sulfur-based chemistry behind activation and binding of a target cysteine was serendipitous.
The antithrombosis factors clopidogrel/Plavix™, ticlopide/Ticlid, etc. are also prodrugs. Activation by cytochrome P450 enzymes results in the scission of a ring carbon-sulfur bond, creating a sulfhydryl group that can then form a disulfide bond with its target cysteine on the adenosine diphosphate (ADP) chemoreceptor P2Y12. In addition to increased specificity for its target, which it permanently inactivates, the active metabolite has improved plasma protein binding characteristics. The thrombosis drugs had their beginnings in functional assays, and fortunately animal studies, because the active metabolite is not produced in most cell-based assays. Both their mechanism of action and target were unknown at the time of discovery.
More recent compounds employing sulfhydryl moieties were rationally designed. Dacomitinib, afatinib, and neratinib are EGFR kinase inhibitors with a high-affinity, nucleotide-analogue moiety that reversibly binds the ATP-binding pockets of numerous kinases and a second moiety designed to covalently bond with a non-conserved cysteine (present in EGFR but not its homologues). The electrophilic moiety is purposefully a low-reactivity acrylamide to minimize off-target reactions. A related chemical approach used low-reactivity, acrylamide-based, electrophiles to target non-conserved (in humans) and non-catalytic-residue cysteine of the HCV NS3/4A viral protease (HCVP).
In sum, all known approaches either minimize the exposure of highly reactive electrophiles (“hiding” reactive sulfur in disulfides or in rings), or minimize the reactivity of exposed electrophiles (using acrylamide adducts). Unfortunately, however, the specificity of sulphenamides depends upon an acidic environment (pH<4.5) found only in the intestinal lumen, and the specificity of therapeutics employing reactive sulfhydryl groups is poorly understood. A few therapeutic molecules were obtained by rationally attaching low-reactivity electrophiles to high affinity and specificity moieties. Unfortunately, compounds with high affinity and specificity tend to appear in the final stages of a drug development effort making this approach best suited for improving existing specificity.
There exists a need for a strategy for conferring specificity to drugs that target cysteine, in general, but pairs of cysteine, in particular.